This invention relates to compression-refrigeration, air conditioning, and heat pump systems in which the refrigerant is compressed in a rotary displacement compressor, the outlet of the compressor is connected to a condenser, the inlet of the compressor is connected to an evaporator, oil is injected into the compressor by an oil injector, oil is separated from the refrigerant and recirculated to the oil injection mechanism, the refrigerant is a fluorinated, non-chlorinated hydrocarbon, and the base lubricant is a polyether polyol or monol.
Numerous studies have linked the presently widely used chlorofluorocarbons with the depletion of the earth's protective ozone layer which protects life on earth from harmful sun rays. It is widely accepted that this depletion of the ozone layer has been the cause of higher skin cancer rates. It is the chlorine in the so-called CFC's, which are currently used in refrigeration and other systems, which reacts with the ozone layer. As a result, by international agreement, the use of highly chlorinated products will be gradually terminated.
One of several refrigerants which has been developed as a substitute refrigerant medium is 1,1,1,2-tetrafluoroethane, also known as refrigerant R-134a, which contains no chlorine or bromine, and does not pose a risk to stratospheric ozone. This refrigerant, which can be manufactured by the method described in U.S. Pat. No. 4,311,863, is thought, accordingly, to be a refrigerant of the future, and it is expected that R-134a will be widely used in oil-flooded rotary displacement compressors.
In the basic, closed, compression refrigeration cycle, liquid refrigerant flows from the condenser under pressure through the expansion valve to the evaporator coils where it evaporates, absorbing heat and cooling the room or other space where cooling is desired. The vapor is then drawn into the compressor where its pressure and temperature are raised. When the hot, high pressure vapor flows from the compressor to the condenser, usually via an oil separator, the condenser cooling liquid removes enough heat from it to condense it. This liquid refrigerant then flows to the evaporator once again. In a single stage system the refrigerant vapor is compressed from suction pressure to condensing pressure in one operation, but multi-stage systems are also utilized in which the vapor is raised to the desired pressure range through a series of consecutive compressions.
Rotary screw compressors employ helical rotary lobe elements in compressing gas, in contradistinction to reciprocating pistons, and operate on the positive displacement principal. The most commonly used rotary screw compressors are of the oil flooded, double helical screw type wherein refrigerant gas is compressed by the action of an intermeshing male and female rotor which turns in a cylinder, the turning rotors drawing gas into the voids formed by the rotor's lobes. As the lobes turn past the intake ports, a charge of gas is trapped and sealed in the adjacent interlobe spaces. As rotation continues, the volume between the discharge end plate of the rotor chamber and the point of rotor mesh decreases. In flooded systems, during compression, oil under pump pressure is sprayed into the cylinder through orifices in the cylinder walls. Compression continues until the end of the female rotor passes over the outlet port so the gas discharges into the system, and normally compression can be accomplished in one stage. Oil is injected directly into the gas stream at the beginning of compression, and intimate contact of the oil spray or mist and gas permits the oil to absorb a considerable amount of the heat of compression.
The lubricant in an oil flooded rotary screw refrigerator system, further, must not only lubricate the bearings and gears, but also aid in sealing the clearance between the screws or lobes and the casing. When, in view of the cooling function which is also to be achieved, a high volume of oil is injected during the compression phase, an important consideration is the diluting effect on viscosity of the dissolved refrigerant. Synthetic lubricants, and particularly certain polygylcols, have been proposed for various chlorinated halocarbon refrigerants, because the final viscosity, under the effects of temperature, pressure, and type of refrigerant, can be higher, even though the solubility of the refrigerant gas in the lubricant is greater, than for a corresponding mineral oil in rotary screw compressors where the sealing effect of the lubricant plays an important role in the overall efficiency. The oils previously proposed for other refrigerants are not suitable for non-chlorinated fluorocarbon refrigerants such as R134a. They too readily dissolve the refrigerant at high temperatures but not at low temperatures. Significant benefits in compressor volumetric efficiency are achievable if the viscosity of the lubricant is not substantially diluted by the refrigerant. Decreased viscosity of the oil film can cause thin film lubrication conditions which result in wear, and affect the ability of the film to adhere to metal surfaces. Further, as indicated, a decreased viscosity provides a less efficient sealing effect so that efficiency losses can be considerable.
The oil-injected compressor also has other efficiency losses when it operates with a lubricant that can dissolve the compressed gas. The lubricant that is injected at an intermediate pressure point in the compressor contains dissolved gas which, when exposed to lower pressure, flashes, and this "free" gas has to be recompressed without doing any real work. Also, some oil within the compressor leaks back to lower pressure regions and, when it contains dissolved gas, this gas boils off and has to be recompressed.
Still further, oil-flooded compressors normally have high pressure oil separators which tend to facilitate the dissolution of the circulated gas refrigerant into the lubricant within the oil separator.